Health and Safety Executive

Potential for leaks of LPG from underground pipes into caravans and park homes

Prepared by the Health and Safety Laboratory for the Health and Safety Executive

RR945 Research Report

Health and Safety Executive

Potential for leaks of LPG from underground pipes into caravans and park homes

Jill Wilday CEng FIChemE Harpur Hill Buxton Derbyshire SK17 9JN

A key factor leading to the fatal explosion at ICL Plastics Ltd was corrosion of underground metallic service pipework carrying LPG into the factory building. Leaking of LPG from the corroded pipe into the building with its subsequent ignition resulted in the explosion. A programme to replace buried metallic service pipework with pipework made from non-corrosive materials such as polyethylene at industrial and commercial premises is currently underway. The replacement of such pipework supplying LPG to communal buildings at Residential Home and Holiday Parks forms part of this programme. This report considers ventilation requirements for caravans and park homes which have a skirt installed to enclose the volume underneath them. The amount of ventilation required is considered in relation to the possibility of LPG leaking into the space under the caravan/park home due to corrosion of any underground LPG service pipe which rises under it.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the author alone and do not necessarily reflect HSE policy.

HSE Books

© Crown copyright 2012

First published 2012

You may reuse this information (not including logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit www.nationalarchives.gov.uk/doc/open-government-licence/, write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email psi@nationalarchives.gsi.gov.uk.

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ACKNOWLEDGEMENTS

Helpful information on caravan and park home dimensions and applicable standards was received from Joan Clark, Deputy Director General, British Holiday & Home Parks Association (BH & HPA) and Jackie Garwen, Director, Chiltern Consulting.

Ceri Petire from HSE provided helpful information on LPG related incidents in caravan and park homes.

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KEY MESSAGES

This report considers ventilation requirements for caravans and park homes which have a skirt installed to enclose the volume underneath them. Ventilation is considered in relation to the possibility of LPG leaking into the space under the caravan / park home due to corrosion of any underground LPG service pipe which rises under it.

If the LPG supply is medium pressure (750 mbar) then ventilation should: • have a total free area of at least 74,000 mm2; • be provided as any number and shape of vents, with an aspect ratio less than 4:1, to give

the total vent free area above; • ideally, include at least two vents on opposite sides of the caravan skirt, and as close as

possible to the location of the LPG pipe; • where the home is sited on sloping ground, ventilation should be provided in the

skirting on the lowest side to ensure adequate ventilation; • take account of the need for drop holes beneath LPG appliances and where buried

service pipes carrying LPG emerge from the ground; • be kept clear of any obstructions to ensure a good air flow.

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EXECUTIVE SUMMARY

Objectives

Static caravans and park homes can have liquefied petroleum gas (LPG) supplied by underground pipe from a central supply tank. The riser exits below the caravan and, if there is an air-tight skirt around the base of the caravan, any leak from the pipe would be into a virtually air-tight volume. If the underground pipe is metallic, there is the potential for leakage due to corrosion. HSE would like to be able to give advice to caravan owners about the degree of ventilation necessary to prevent explosion in such an event.

Main findings

Making the assumption of perfect mixing in the space, which is below a caravan or park home and enclosed by a skirt, the size of vent required to prevent an explosive atmosphere is independent of the size of caravan, and depends only on the flow rate of LPG from a corrosion leak.

The assumption of perfect mixing is reasonable if

a) A safety factor according to BS EN 60079-10 is used, and

b) The vents are placed adjacent to the LPG pipe.

No incidents have been identified in which a fire or explosion in a caravan or park home was caused by a corrosion leak in the underground LPG pipe. However, incidents have occurred which involved appliances and LPG cylinders.

Ventilation specified in the Building Regulations exceeds that which would be required for corrosion leakage of low pressure (LP) LPG supply pipes. However, for medium pressure (MP) LPG, additional ventilation would need to be provided.

Recommendations

For MP LPG, vents with a total free area of at least 74,000 mm2 should be provided. Any convenient number and shape, with an aspect ratio less than 4:1, could be chosen to provide a free area of 74,000 mm2. Ideally, at least two vents should be provided on opposite sides of the caravan skirt, including vents as close as possible to the location of the LPG pipe.

For LP LPG, vents with a total free area of at least 7400 mm2 are required. Again, ideally two such vents should be provided on either side of the caravan skirt, as close as possible to the location of the LPG pipe. However, if ventilation has already been provided in accordance with the Building Regulations (ODPM, 2004) then this will exceed the ventilation requirement for corrosion leakage of LP LPG and should be sufficient.

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CONTENTS PAGE

KEY MESSAGES..............................................................................III

EXECUTIVE SUMMARY ................................................................. V

1. INTRODUCTION..........................................................................1

2. IMPLICATIONS............................................................................2

3. METHODOLOGY.........................................................................3 3.1 INFORMATION ABOUT CARAVAN AND PARK HOME DIMENSIONS 3 3.2 ESTIMATION OF REQUIRED VENTILATION RATE 4

3.2.1 Required air changes per hour..................................................................................... 4 3.2.2 Air vent requirements .................................................................................................. 6

3.3 ACCURACY OF THE PROPOSED METHODOLOGY 6 3.3.1 Assumption of perfect mixing..................................................................................... 6 3.3.2 Use of BS EN 60079-10 safety factors ....................................................................... 7 3.3.3 Incident data ................................................................................................................. 7

4. RESULTS.....................................................................................9 4.1 REQUIRED AIR CHANGES PER HOUR 9 4.2 SIZE OF VENT REQUIRED 9

5. CONCLUSIONS AND RECOMMENDATIONS.........................11 5.1 CONCLUSIONS 11 5.2 RECOMMENDATIONS 11

6. REFERENCES...........................................................................12

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1. INTRODUCTION

The incident at ICL Plastics Ltd, Glasgow in May 2004 occurred when a leak of gas from a corroded, buried steel service pipe into the basement of a factory led to an accumulation of gas and an explosion. The explosion caused the building to collapse killing nine people and injuring 33, some critically. This highlighted the possibility that corrosion leakage of LPG from underground service pipes from an external LPG tank to the property have the potential for LPG to migrate into a property and possibly give rise to a flammable atmosphere.

In response to this incident, a Public Inquiry was held in July 2008, chaired by Lord Gill, and made a number of recommendations1 regarding the use of buried metallic service LPG pipework. In particular, to reduce the risk of another incident of this type, underground metallic pipework at industrial and commercial premises should be replaced with pipework made from materials not subject to corrosion, such as polyethylene (PE). As a result of this recommendation, UKLPG, the LPG suppliers and the HSE have been working together to prioritise underground metallic LPG pipework for replacement. The replacement of the pipework at Residential (Mobile) Home Parks and Holiday Parks forms part of this programme.

In order to inform HSE’s approach to domestic premises that lie outside the industrial/commercial replacement programme, a risk-based model was developed by TTAC Ltd (Hunt et al, 2010) for ingress from corrosion leaks from corroded underground metallic LPG pipes into domestic properties. The model for houses with suspended floors developed by TTAC Ltd is of most relevance to caravans. TTAC Ltd showed that, if the concentration of LPG in the void space below the floor exceeds the lower explosive limit (LEL), then either: • Ignition in the void space is possible (but unlikely); • “It is credible that LPG might build up for days or weeks below a floor and then be “sucked

up” into the property following a change in atmospheric conditions or householder activity”.

Static caravans and park homes can have an LPG supplied by underground service pipework from a central supply tank. The riser exits below the caravan and, if there is an air-tight skirt around the base of the caravan, any leak is into a virtually air-light volume. HSE would like to be able to give advice to caravan owners about the degree of ventilation necessary to prevent an explosion in such an event.

The main path between the void under the caravan and the caravan itself is likely to be poorly sealed penetrations such as for drains, waste pipes and the supplies of water, LPG and electricity.

1 www.theiclinquiry.org/ 1

2. IMPLICATIONS

This report considers ventilation requirements for caravans and park homes, which have a skirt installed to enclose the volume underneath them. Ventilation is considered in relation to the possibility of LPG leaking into the space under the caravan / park home, due to corrosion of any underground metallic LPG service pipe which rises under it.

The work uses a simple modelling approach that is considered good practice for hazardous area classification to define the extent of zones, which can potentially contain flammable atmospheres. This approach is considered a reasonable basis to allow HSE to provide guidance to the owners of caravans and park homes.

The following guidance is recommended:

• For MP LPG, vents with a total free area of at least 74,000 mm2 should be provided. Any convenient number and shape, with an aspect ratio less than 4:1, could be chosen to provide a free area of 74,000 mm2. Ideally, at least two vents should be provided on opposite sides of the caravan skirt, including vents as close as possible to the location of the LPG pipe.

• For LP LPG, vents with a total free area of at least 7400 mm2 are required. Again, ideally two such vents should be provided on either side of the caravan skirt, as close as possible to the location of the LPG pipe. However, if ventilation has already been provided in accordance with the Building Regulations (ODPM, 2004) then this will exceed the ventilation requirement for corrosion leakage of LP LPG and should be sufficient.

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3.1

3. METHODOLOGY

INFORMATION ABOUT CARAVAN AND PARK HOME DIMENSIONS

Information was provided by Clark (2011) about the dimensions of typical caravans and park homes. These are reproduced in Table 1 and Figure 1. Table 1 also includes the dimensions assumed for a caravan in the spreadsheet produced by TTAC Ltd in support of their report (Hunt et al, 2010).

Table 1: Dimensions of typical caravans and park homes

Case Length (m) Width (m)

Maximum size of caravan (Caravan Sites Act: 1968) 20 6.8 BS 3632:2005 park home / mobile home / leisure lodge (assume 50 x 20 ft)

15.2 6.1

BS 3632:2005 smaller park home (assume 36 x 10 ft)

11 3

BS EN 1647:2004 holiday caravan / caravan holiday home (assume 35 x 12.25 feet)

10.7 3.7

BS EN 1647:2004 smaller caravan holiday home (assume 28 x 10 feet)

8.5 3

TTAC report caravan dimensions 9.1 3

(a) (b)

(c) (d)

Figure 1: Illustration of typical caravans and park homes

(a) and (b) examples of park home / mobile home built to BS3632; (c) smaller park home built to BS3632; (d) holiday caravan / caravan holiday home built to BS EN1647

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An internet search was carried out for examples of caravan skirting. Figure 2 shows a caravan with rigid PVC skirting which incorporates ventilation grilles.

Figure 2: Example of caravan with rigid skirting (Fensys, 2006)

3.2 ESTIMATION OF REQUIRED VENTILATION RATE

3.2.1 Required air changes per hour

The current work focuses on prevention of a concentration of LPG in the void below the caravan which exceeds the LEL. The LEL for propane is 2.2 % by volume. It is usual to work with a fraction of the LEL, e.g. 0.25 or 0.5, as a criterion (see also section 3.3.2 below). However, the LEL itself has been used here to define a minimum required number of air changes per hour (ACPH), with the average air changes per hour being considerably higher than this.

Calculations have been carried out assuming that the void space is essentially completely mixed (see section 3.4 below). This assumption is not considered unreasonable given that the aim is to provide ventilation of the void space.

Figure 3 is a diagram showing the skirted void under a caravan, which helps define the following terms used in the calculation:

• Q (m3/h) is the volumetric flow of air into the space below the caravan; • Qg (m3/h) is the volumetric leak rate of LPG into the space below the caravan;

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V (m3) is the volume below the caravan and enclosed by the skirt.

Q m3/h of air

Qg m3/h of LPG

(Q + Qg) m3/h air + LPG

Volume V m3

Figure 3: Diagram of the skirted void space under a caravan

The number of air changes per hour is given by Q/V.

Concentration of LPG in the void space = Qg/ (Q + Qg) (volume fraction) (1)

To prevent the LPG concentration in the void exceeding the LEL requires:

Qg/ (Q + Qg) < 0.022 (2)

TTAC Ltd (Hunt et al, 2010) had undertaken the collection and analysis of information on LPG suppliers’ experience of leaks, to understand the frequency and characteristics of relevant LPG leaks at domestic premises. They had drawn conclusions about the maximum flow rates of LPG from corrosion holes in underground service pipes. These were:

• 1 m3/h for medium pressure (MP) (750 mbar) LPG, and • 0.1 m3/h for low pressure (LP) (37 mbar) LPG.

For LPG at MP, as stated above, the leak rate of LPG could be a maximum of 1 m3/h (Hunt et al, 2010), and therefore:

Qg/ (Q + Qg) = 0.022 (3)

1/ (Q + 1) = 0.022 (4)

Q = 44.5 m3/h (5)

The spreadsheet prepared by TTAC Ltd suggests that a typical caravan has a floor area of 10 ft x 30 ft and a void height of 0.6 metres. This has been used as an example for the calculation. Void volume V for this case is therefore 10 x 30 x (0.3048)2 x 0.6 = 16.7 m3.

Required air changes per hour (F) = Q/V = 44.5/16.7 = 2.7 (6)

For pipework carrying LPG at LP , the maximum leak rate was estimated as 0.1 m3/h rather than 1 m3/h, so a minimum ventilation rate of 0.3 ACPH would be sufficient for the example caravan.

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3.2.2 Air vent requirements

The TTAC report (Hunt, et al, 2009) quotes (on page 105) Building Regulations as requiring 500 mm2 of vent per m2 of floor area, which corresponds to a minimum of just under 2 ACPH for a crawl space. TTAC Ltd states that the average would be considerably higher. Closer reading of the Building Regulations (ODPM, 2004) however indicates that twice this area is required, with one opening of this size on each of two opposing walls. The flow of air would be expected to be proportional to the flow area. This relationship between the vent area required and the required air changes per hour (F) can be stated more generally as:

Avent = 500 (F/2) Aplan (7)

where Avent is the required area of each of the two vents provided, F is the number of air changes per hour required, and Aplan is the plan area of the caravan.

Substituting for F from equation (6) and noting that V = Aplan x H, where H is the height of the enclosed crawl space under the caravan, gives:

Avent = (500/2). (Q/H) (8)

i.e. the required vent area depends only on the height of the caravan from the ground, not on its plan area. The height is expected to be relatively uniform and a height of 0.6 metres has been assumed. Equation 5 shows that for MP LPG, Q = 44.5 m3/h. The required vent area is therefore:

Avent = (500/2). (44.5/0.6) = 18500 mm2. (9)

It is therefore apparent that the area of the air vent required, according to the methodology adopted here, is independent of the plan area of the caravan. The vent area calculated from equation (9) is for MP LPG and is based on prevention of a concentration equal to the LEL in the space under the caravan. Twice this area should be provided based on the Building Regulations (ODPM, 2004). However see the consideration of safety factors below.

3.3 ACCURACY OF THE PROPOSED METHODOLOGY

3.3.1 Assumption of perfect mixing

The methodology in section 3.2.1 makes the assumption that the space under the caravan is completely mixed, such that the concentration is uniform throughout the volume. This assumption is made in the methodology presented in the IChemE guidance on hazardous area classification (Cox, Lees & Ang, 1990). However, it is common to calculate the distance to a concentration of 0.5 x LEL to take account of uncertainties in the dispersion modelling. This is reflected by the British Standard (BS EN 60079-10) on hazardous area classification which introduces safety factors to take some account of the uncertainty introduced by the assumption of perfect mixing (see section 3.3.2 below).

The assumption of perfect mixing is considered reasonable when applied to the space beneath a caravan because:

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• There are no internal partitions of the space beneath the caravan. This will almost always be the case because of the need for transport of the caravan by road using its wheels.

• There is a lack of sources of ignition in the space under a caravan, so that long timescales can be expected, allowing steady-state to be achieved, before ignition. Ignition is only likely to occur if a flammable atmosphere extends into the caravan itself, e.g. via gaps in sealing service pipes, drains etc.

Nevertheless, the assumption of perfect mixing will not be completely true and there is likely to be a region close to the release point at which the concentration is higher than the average for the space beneath the caravan. The shape of the area beneath the caravan (long in comparison with its height) also does not facilitate perfect mixing. This can however be mitigated by positioning the vents as close as possible to where the LPG pipe leaves the ground and enters the caravan.

It is not simple to depart from the assumption of perfect mixing, except by use of the safety factors. To obtain accuracy beyond this would require the use of computational fluid dynamics (CFD) modelling and would need to be specific to the geometry of each installation. This would be beyond the level of accuracy that is considered normal good practice for hazardous area classification.

3.3.2 Use of BS EN 60079-10 safety factors

It would be good practice to adopt a safety factor, as discussed in Annex B (informative) to BS EN 60079-10.

Potential releases are graded as ‘continuous’, ‘primary’ or ‘secondary’, where a continuous grade of release normally leads to a zone 0, a primary grade to zone 1 and a secondary grade to zone 2. Release of LPG due to pipe corrosion would be a secondary grade of release as it will be very infrequent. This is supported by information about leak frequencies collected in the TTAC report (Hunt et al, 2010).

For a secondary release, BS EN 60079-10 Annex B recommends applying a safety factor of 2 to the lower explosive limit, i.e. to base calculations on a concentration of 0.5 LEL rather than the LEL. This has the effect of doubling the required air changes per hour, and doubling the required vent area. The results presented in section 4 below incorporate this safety factor.

3.3.3 Incident data

Gas supply industry representatives commented that the assumption of perfect mixing was unlikely to be valid and anecdotal evidence suggested there had been incidents which had resulted in fire / explosion. However when approached for additional detail, no LPG supplier was able to provide details of any incidents of LPG building up to explosive limits beneath a caravan as a result of a failed metallic LPG service pipe.

The following attempts have been made to identify such incidents:

• Request to the gas supply industry representatives to supply details of incidents. Details of only one such incident were supplied. On follow-up (informal discussion with one of the investigators), this was found to involve a pressurised leak from an LPG gas bottle, not a corrosion leak from a buried pipe.

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• Search of RIDDOR2 reports. This failed to locate any incidents in which a caravan fire/ explosion had been due to a corrosion leak in an underground metallic service pipe.

• Literature search. This focussed on newspaper reports. Five reports were located which referred to caravan explosions but none gave sufficient information to be able to tell whether the source was corrosion leakage from an underground pipe.

• Request to HSE process safety specialists. Only one incident was identified and this involved failure of a polyethylene pipe, not corrosion leakage from a metal pipe.

It is therefore concluded that there is no evidence of caravan fire and explosion incidents resulting from corrosion leaks from buried metallic LPG service pipes. This supports the position that existing ventilation in caravan skirts has been adequate to prevent ignition of LPG from any corrosions leaks that may have occurred.

It should be noted that incidents have been identified relating to LPG leaks from appliances within the caravan and / or cylinders resulting in an explosive limit being reached.

2 Reports of incidents under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 1995 8

4.1

4.2

4. RESULTS

REQUIRED AIR CHANGES PER HOUR

The method described in section 3.2.1, together with the safety factor described in 3.3.2, has been used to consider different sizes of caravans and residiential park homes (Clark, 2011). Results are shown in Table 2 below.

Table 2: Ventilation requirements for different sizes of caravan and residential park home

Case Length (m) Width (m) Ventilation required (ACPH)

MP LPG LP LPG Maximum size of caravan (Caravan Sites Act: 1968)

20 6.8 1.1 0.1

BS 3632:2005 park home / mobile home / leisure lodge (assume 50 x 20 ft)

15.2 6.1 1.60 0.16

BS 3632:2005 smaller park home (assume 36 x 10 ft)

11 3 4.5 0.45

BS EN 1647:2004 holiday caravan / caravan holiday home (assume 35 x 12.25 ft)

10.7 3.7 3.74 0.37

BS EN 1647:2004 smaller caravan holiday home (assume 28 x 10 ft)

8.5 3 5.82 0.58

TTAC caravan dimensions

9.1 3 5.44 0.54

Note: This Table is based on the same size of LPG leak in all cases. The same number of air changes per hour (ACPH) gives a larger air flow for larger caravans. This accounts for higher ACPH being required for smaller caravans compared with larger caravans.

SIZE OF VENT REQUIRED

As discussed in section 3.2.2, the size of the vent is independent of the size of the caravan.

For MP LPG, section 3.2.2 derives a vent area of 18500 mm2. Applying a safety factor from BS EN 60079-10 increases this to 37000 mm2. Assuming a typical free area of a ventilation grille of 0.7, this equates to a square grille of side 230 mm (9 inches). However, any convenient shape with an aspect ratio less than 4:1 could be chosen to provide a free area of 37000 mm2. Twice this area should be provided based on the Building Regulations (ODPM, 2004) giving a total required area of 74000 mm2. For example, ideally this could be achieved by two such vents on either side of the caravan skirt. They should be placed on opposite sides of the skirt as close as possible to the location of the LPG pipe.

For LP LPG, a vent free area of 3700 mm2 is required. This equates to a square grille of side 73 mm (approximately 3 inches). Again, two such vents should be provided on either side of the

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caravan skirt. They should be placed on opposite sides of the skirt as close as possible to the location of the LPG pipe. However, if ventilation has already been provided in accordance with the Building Regulations (ODPM, 2004), then this will exceed the ventilation requirement for corrosion leakage of LP LPG.

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5. CONCLUSIONS AND RECOMMENDATIONS

5.1 CONCLUSIONS

Making the assumption of perfect mixing in the space, which is below a caravan or park home and enclosed by a skirt, the size of vent required to prevent an explosive atmosphere is independent of the size of caravan, and depends only on the flow rate of LPG from a corrosion leak.

The assumption of perfect mixing is reasonable if

a) A safety factor according to BS EN 60079-10 is used, and

b) The vents are placed adjacent to the LPG pipe.

No incidents have been identified in which a fire or explosion in a caravan or park home was caused by a corrosion leak in the underground LPG pipe. However, it should be noted that incidents have been identified relating to LPG leaks from appliances within the caravan and / or cylinders resulting in an explosive limit being reached.

Ventilation specified in the Building Regulations exceeds that which would be required for corrosion leakage of low pressure (LP) LPG supply pipes. However, for medium pressure (MP) LPG, additional ventilation would need to be provided.

5.2 RECOMMENDATIONS

For MP LPG, vents with a total free area of at least 74,000 mm2 should be provided. Any convenient number and shape, with an aspect ratio less than 4:1, could be chosen to provide a free area of 74,000 mm2. Ideally, at least two vents should be provided on opposite sides of the caravan skirt, including vents as close as possible to the location of the LPG pipe.

For LP LPG, vents with a total free area of at least 7400 mm2 are required. Again, ideally two such vents should be provided on either side of the caravan skirt, as close as possible to the location of the LPG pipe. However, if ventilation has already been provided in accordance with the Building Regulations (ODPM, 2004) then this will exceed the ventilation requirement for corrosion leakage of LP LPG and should be sufficient.

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6. REFERENCES

BSI (2009), Explosive atmospheres. Classification of areas, BS EN 60079-10

Clark J (2011), email to Ceri Petrie, HSE, dated 14/1/2011

Fensys (2006), Fensys Caravan Skirting, information leaflet CP/CP/12/06, www.fensys.co.uk

Hunt M, Somaiya K & Taig T, (2010), “Risk assessment of corrosion leakage of LPG from domestic underground service pipework: Main Report”, TTAC Ltd, March 2010

ODPM (2004), The Building Regulations 2000 Part C, Site preparation and resistance to contaminants and moisture, 2004 Edition

Published by the Health and Safety Executive 11/12

Health and Safety Executive

Potential for leaks of LPG from underground pipes into caravans and park homes A key factor leading to the fatal explosion at ICL Plastics Ltd was corrosion of underground metallic service pipework carrying LPG into the factory building. Leaking of LPG from the corroded pipe into the building with its subsequent ignition resulted in the explosion. A programme to replace buried metallic service pipework with pipework made from non-corrosive materials such as polyethylene at industrial and commercial premises is currently underway. The replacement of such pipework supplying LPG to communal buildings at Residential Home and Holiday Parks forms part of this programme. This report considers ventilation requirements for caravans and park homes which have a skirt installed to enclose the volume underneath them. The amount of ventilation required is considered in relation to the possibility of LPG leaking into the space under the caravan/park home due to corrosion of any underground LPG service pipe which rises under it.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the author alone and do not necessarily reflect HSE policy.

RR945

www.hse.gov.uk